Method and apparatus for triggering uplink data in wireless lan

ABSTRACT

A method and an apparatus for triggering uplink data in a wireless LAN are disclosed. A method for receiving uplink data in a wireless LAN may comprise: a step in which an access point (AP) transmits a trigger frame for random connection; and a step in which the AP receives at least one buffer status report frame from at least one STA on a random connection transmission opportunity (TXOP) which is set on the basis of the trigger frame for random connection, wherein the random connection TXOP is allocated for a channel access attempt for transmitting the buffer status report frame of each of a plurality of non-specific STAs, and each of the at least one STA may be an STA which succeeds in the channel access among the plurality of non-specific STAs.

BACKGROUND OF THE INVENTION Field of the Invention

The present invention relates to wireless communication, and moreparticularly, to a method and apparatus for triggering uplink data in awireless local area network (WLAN).

Related Art

Discussion for a next-generation wireless local area network (WLAN) isin progress. In the next-generation WLAN, an object is to 1) improve aninstitute of electronic and electronics engineers (IEEE) 802.11 physical(PHY) layer and a medium access control (MAC) layer in bands of 2.4 GHzand 5 GHz, 2) increase spectrum efficiency and area throughput, 3)improve performance in actual indoor and outdoor environments such as anenvironment in which an interference source exists, a denseheterogeneous network environment, and an environment in which a highuser load exists, and the like.

An environment which is primarily considered in the next-generation WLANis a dense environment in which access points (APs) and stations (STAs)are a lot and under the dense environment, improvement of the spectrumefficiency and the area throughput is discussed. Further, in thenext-generation WLAN, in addition to the indoor environment, in theoutdoor environment which is not considerably considered in the existingWLAN, substantial performance improvement is concerned.

In detail, scenarios such as wireless office, smart home, stadium,Hotspot, and building/apartment are largely concerned in thenext-generation WLAN and discussion about improvement of systemperformance in a dense environment in which the APs and the STAs are alot is performed based on the corresponding scenarios.

In the next-generation WLAN, improvement of system performance in anoverlapping basic service set (OBSS) environment and improvement ofoutdoor environment performance, and cellular offloading are anticipatedto be actively discussed rather than improvement of single linkperformance in one basic service set (BSS). Directionality of thenext-generation means that the next-generation WLAN gradually has atechnical scope similar to mobile communication. When a situation isconsidered, in which the mobile communication and the WLAN technologyhave been discussed in a small cell and a direct-to-direct (D2D)communication area in recent years, technical and business convergenceof the next-generation WLAN and the mobile communication is predicted tobe further active.

SUMMARY OF THE INVENTION

The present invention provides a method of triggering uplink data in awireless local area network (WLAN).

The present invention also provides an apparatus for performing a methodof triggering uplink data in a WLAN.

To achieve the aforementioned purpose of the present invention, oneaspect of the present invention may provide a method of receiving uplinkdata in a WLAN. The method may include transmitting, by an access point(AP), a trigger frame for random access, and receiving, by the AP, atleast one buffer status report frame from at least one station (STA) ona random access transmission opportunity (TXOP) configured based on thetrigger frame for random access. The random access TXOP may be allocatedto attempt channel access for transmission of the buffer status reportframe of each of a plurality of unspecific STAs. Each of the at leastone STA may be an STA which has succeeded in the channel access amongthe plurality of unspecific STAs.

To achieve the aforementioned purpose of the present invention, anotheraspect of the present invention may provide an AP for receiving uplinkdata in a WLAN. The AP may include a radio frequency (RF) unitconfigured to transmit or receive a radio signal, and a processoroperatively coupled to the RF unit. The processor may be configured totransmit a trigger frame for random access, and receive at least onebuffer status report frame from at least one STA on a random access TXOPconfigured based on the trigger frame for random access. The randomaccess TXOP may be allocated to attempt channel access for transmissionof the buffer status report frame of each of a plurality of unspecificSTAs. Each of the at least one STA may be an STA which has succeeded inthe channel access among the plurality of unspecific STAs.

Uplink (UL) multi-user (MU) transmission may be supported on the basisof triggering of uplink data of an access point (AP). Therefore, anefficiency and throughput of a wireless local area network (WLAN) can beimproved.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a conceptual view illustrating a structure of a wireless localarea network (WLAN).

FIG. 2 is a conceptual view illustrating a scanning method in a WLAN.

FIG. 3 is a conceptual view illustrating an authentication procedure andan association procedure being performed after a scanning procedure ofan access point (AP) and a station (STA).

FIG. 4 is a conceptual view illustrating an uplink (UL) multi-user (MU)transmission method according to an embodiment of the present invention.

FIG. 5 is a conceptual view illustrating a UL MU transmission procedureaccording to an embodiment of the present invention.

FIG. 6 is a conceptual view illustrating buffer status report STAdetermination information according to an embodiment of the presentinvention.

FIG. 7 is a conceptual view illustrating a method of triggering a bufferstatus report frame of an STA according to an embodiment of the presentinvention.

FIG. 8 is a conceptual view illustrating a method of triggering a bufferstatus report frame of an STA according to an embodiment of the presentinvention.

FIG. 9 is a conceptual view illustrating a downlink (DL) MU physicallayer (PHY) protocol data unit (PPDU) format according to an embodimentof the present invention.

FIG. 10 is a conceptual view illustrating transmission of a UL MU PPDUaccording to an embodiment of the present invention.

FIG. 11 is a block diagram illustrating a wireless device according toan embodiment of the present invention.

DESCRIPTION OF EXEMPLARY EMBODIMENTS

FIG. 1 is a conceptual view illustrating the structure of a wirelesslocal area network (WLAN).

An upper part of FIG. 1 shows the structure of the institute ofelectrical and electronic engineers (IEEE) 802.11 infrastructurenetwork.

Referring to the upper part of FIG. 1, the WLAN system may include oneor more basic service sets (BSSs) 100 and 105. The BSS 100 or 105 is aset of an access point (AP) such as AP 125 and a station (STA) such asSTA1 100-1 that may successfully sync with each other to communicatewith each other and is not the concept to indicate a particular area.The BSS 105 may include one AP 130 and one or more STAs 105-1 and 105-2connectable to the AP 130.

The infrastructure BSS may include at least one STA, APs 125 and 130providing a distribution service, and a distribution system (DS) 110connecting multiple APs.

The distribution system 110 may implement an extended service set (ESS)140 by connecting a number of BSSs 100 and 105. The ESS 140 may be usedas a term to denote one network configured of one or more APs 125 and130 connected via the distribution system 110. The APs included in oneESS 140 may have the same service set identification (SSID).

The portal 120 may function as a bridge that performs connection of theWLAN network (IEEE 802.11) with other network (for example, 802.X).

In the infrastructure network as shown in the upper part of FIG. 1, anetwork between the APs 125 and 130 and a network between the APs 125and 130 and the STAs 100-1, 105-1, and 105-2 may be implemented.However, without the APs 125 and 130, a network may be establishedbetween the STAs to perform communication. The network that isestablished between the STAs without the APs 125 and 130 to performcommunication is defined as an ad-hoc network or an independent basicservice set (BSS).

A lower part of FIG. 1 is a concept view illustrating an independentBSS.

Referring to the lower part of FIG. 1, the independent BSS (IBSS) is aBSS operating in ad-hoc mode. The IBSS does not include an AP, so thatit lacks a centralized management entity. In other words, in the IBSS,the STAs 150-1, 150-2, 150-3, 155-4 and 155-5 are managed in adistributed manner. In the IBSS, all of the STAs 150-1, 150-2, 150-3,155-4 and 155-5 may be mobile STAs, and access to the distributionsystem is not allowed so that the IBSS forms a self-contained network.

The STA is some functional medium that includes a medium access control(MAC) following the Institute of Electrical and Electronics Engineers(IEEE) 802.11 standards and that includes a physical layer interface forradio media, and the term “STA” may, in its definition, include both anAP and a non-AP station (STA).

The STA may be referred to by various terms such as mobile terminal,wireless device, wireless transmit/receive unit (WTRU), user equipment(UE), mobile station (MS), mobile subscriber unit, or simply referred toas a user.

Hereinafter, in an embodiment of the present invention, data (or aframe) to be transmitted from the AP to the STA may be expressed by theterm ‘downlink data (or a downlink frame)’, and data (or a frame) to betransmitted from the STA to the AP may be expressed by the term ‘uplinkdata (or an uplink frame)’. In addition, transmission from the AP to theSTA may be expressed by the term ‘downlink transmission’, andtransmission from the STA to the AP may be expressed by the term ‘uplinktransmission’.

Further, a physical layer (PHY) protocol data unit (PPDU), frame, anddata transmitted through downlink transmission may be expressedrespectively by the term “downlink PPDU”, “downlink frame”, and“downlink data”. The PPDU may be a data unit including a PPDU header anda physical layer service data unit (PSDU) (or a MAC protocol data unit(MPDU)). The PPDU header may include a PHY header and a PHY preamble,and the PSDU (or MPDU) may include a frame (or an information unit of aMAC layer) or may be a data unit indicating the frame. The PHY headermay be expressed by the term “physical layer convergence protocol (PLCP)header”, and the PHY preamble may be expressed by the term “PLCPpreamble”.

Further, a PPDU, frame, and data transmitted through uplink transmissionmay be expressed respectively by the term “uplink PPDU”, “uplink frame”,and “uplink data”.

In the conventional WLAN system, a full bandwidth is used for downlinktransmission to one STA and uplink transmission to one STA on the basisof single (SU)-orthogonal frequency division multiplexing (OFDM)transmission. Further, in the conventional WLAN system, an AP mayperform downlink (DL) multi-user (MU) transmission on the basis of MUmultiple input multiple output (MIMO), and such transmission may beexpressed by the term “DL MU MIMO transmission”.

In a WLAN system according to an embodiment of the present invention, atransmission method based on orthogonal frequency division multipleaccess (OFDMA) may be supported for uplink transmission and downlinktransmission. More specifically, in the WLAN system according to theembodiment of the present invention, an AP may perform DL MUtransmission based on OFDMA, and such transmission may be expressed bythe term “DL MU OFDMA transmission”. When the DL MU OFDMA transmissionis performed, the AP may transmit downlink data (or downlink frame,downlink frame PPDU) to each of a plurality of STAs on overlapping timeresources through a plurality of frequency resources (a plurality ofsubbands (or subchannels)). The DL MU OFDMA transmission may be usedtogether with the DL MU MIMO transmission. For example, DL MU-MIMOtransmission based on a plurality of space-time streams (or spatialstreams) may be performed on a specific subband (or subchannel)allocated for the DL MU OFDMA transmission.

Further, in the WLAN system according to the embodiment of the presentinvention, uplink (UL) multi-user (MU) transmission may be supported sothat the plurality of STAs transmit data to the AP on the same timeresource. Uplink transmission on an overlapping time resource may beperformed by each of the plurality of STAs on a frequency domain or aspatial domain.

When the uplink transmission is performed by each of the plurality ofSTAs on the frequency domain, a different frequency resource (subband,subchannel, or resource unit (RU)) may be allocated for each of theplurality of STAs as an uplink transmission resource. Each of theplurality of STAs may transmit uplink data to the AP through theallocated different frequency resource. A transmission method throughthe different frequency resource may be expressed by the term “UL MUOFDMA transmission method”.

When the uplink transmission is performed by each of the plurality ofSTAs on the spatial domain, a different space-time stream (or spatialstream) may be allocated to each of the plurality of STAs, and each ofthe plurality of STAs may transmit uplink data to the AP through thedifferent space-time stream. A transmission method through the differentspatial stream may be expressed by the term “UL MU MIMO transmissionmethod”.

UL MU OFDMA transmission and UL MU MIMO transmission may be performedtogether. For example, UL MU MIMO transmission based on a plurality ofspace-time streams (or spatial streams) may be performed on a specificsubband (or subchannel) allocated for UL MU OFDMA transmission.

The UL MU transmission may be performed by using triggering of the AP.The AP needs to pre-acquire information on pending (or buffered) uplinkdata in an associated STA to trigger UL MU transmission of the pluralityof STAs. Further, the AP may also determine whether to trigger UL MUtransmission by considering a state of a BSS load to improve WLANefficiency.

FIG. 2 is a conceptual view illustrating a scanning method in a wirelessLAN.

Referring to FIG. 2, the scanning method may be divided into passivescanning 200 and active scanning 250.

Referring to the left side of FIG. 2, passive scanning 200 may beperformed by a beacon frame 230, which is periodically broadcasted by anAP 210. The AP 210 of the wireless LAN broadcasts a beacon frame 230 toa non-AP STA 240 at each specific interval (e.g., 100 msec). Informationon the current network may be included in the beacon frame 230. Byreceiving the beacon frame 230 that is periodically broadcasted, thenon-AP STA 240 receives the network information and may perform scanningon the AP 240, which will be performing the authentication/associationprocedure, and the channel.

The passive scanning method 200 may be performed by simply receiving thebeacon frame 230 that is being transmitted from the AP 210 withoutrequiring the non-AP STA 240 to transmit any frames. Therefore, thepassive scanning 200 is advantageous in that the overall overhead, whichoccurs due to the transmission/reception of data within the network, issmall. However, since the scanning process can only be performedmanually in proportion to the cycle of the beacon frame 230, passivescanning 200 is disadvantageous in that the time consumed for performingthe scanning process is relatively longer in comparison with the activescanning method. Detailed description on the beacon frame is disclosedin 8.3.3.2 beacon frame of the IEEE Draft P802.11-REVmb™/D12, November2011 ‘IEEE Standard for Information Technology Telecommunications andinformation exchange between systems—Local and metropolitan areanetworks—Specific requirements Part 11: Wireless LAN Medium AccessControl (MAC) and Physical Layer (PHY) Specifications (hereinafterreferred to as IEEE 802.11)’, which was disclosed in November 2011. InIEEE 802.11 ai, a beacon frame of another format may be additionallyused, and such beacon frame may be referred to as a fast initial linksetup (FILS) beacon frame. Additionally, a measurement pilot frame maybe used in the scanning procedure as a frame including only a portion ofthe information of the beacon frame. The measurement pilot frame isdisclosed in the IEEE 802.11 8.5.8.3 measurement pilot format.

Referring to the right side of FIG. 2, in active scanning 250, a non-APSTA 290 may transmit a probe request frame 270 to an AP 260, therebybeing capable of actively performing a scanning procedure.

After receiving the probe request frame 270 from the non-AP STA 290, theAP 260 waits for a random period of time in order to prevent framecollision. And, then, the AP 260 may include network information to aprobe response frame 280 and may transmit the probe response frame 280to the non-AP STA 290. The non-AP STA 290 may obtain the networkinformation based on the received probe response frame 280 and may thenstop the scanning procedure.

In case of active scanning 250, since the non-AP STA 290 activelyperforms scanning, it is advantageous in that the time consumed forperforming the scanning procedure is short. However, since the non-APSTA 290 is required to transmit the probe request frame 270, it isdisadvantageous in that the network overhead increases for thetransmission and reception of the frames. The probe request frame 270 isdisclosed in IEEE 802.11 8.3.3.9, and the probe response frame 280 isdisclosed in IEEE 802.11 8.3.3.10.

Once the scanning is completed, the AP and the non-AP STA may performthe authentication and association procedures.

FIG. 3 is a conceptual view illustrating an authentication procedure andan association procedure being performed after the scanning procedure ofthe AP and the STA.

Referring to FIG. 3, after performing the passive/active scanningprocedure, the authentication procedure and the association proceduremay be performed with one of the scanned APs.

The authentication and association procedures may be performed, forexample, through 2-way handshaking. The left side of FIG. 3 is aconceptual view showing the authentication and association proceduresafter performing passive scanning, and the right side of FIG. 3 is aconceptual view showing the authentication and association proceduresafter performing scanning.

Regardless of whether the active scanning method or the passive scanningmethod has been used, the authentication procedure and the associationprocedure may be equally performed by exchanging an authenticationrequest frame 310/authentication response frame 320 and an associationrequest frame 330/association response frame 340 between the AP 300 and350 and the non-AP STA 305 and 355.

During the authentication procedure, the non-AP STA 305 and 355 maytransmit an authentication request frame 310 to the AP 300 and 350. As aresponse to the authentication request frame 310, the AP 300 and 350 maytransmit an authentication response frame 320 to the non-AP STA 305 and355. Detailed description on the authentication frame format isdisclosed in IEEE 802.11 8.3.3.11.

During the association procedure, the non-AP STA 305 and 355 maytransmit an association request frame 330 to the AP 300 and 350. And, asa response to the association request frame 330, the AP 300 and 350 maytransmit an association response frame 340 to the non-AP STA 305 and355. Information related to the ability of the non-AP STA 305 and 355 isincluded in the association request frame 330, which is transmitted tothe AP. Based on the capability information of the non-AP STA 305 and355, the AP 300 and 350 may determine whether or not the non-AP STA 305and 355 can be supported. In case the non-AP STA 305 and 355 can besupported, the AP 300 and 350 may transmit the association responseframe 340 to the non-AP STA 305 and 355. The association response frame340 may include information on whether or not the association requestframe 330 is accepted and the corresponding reason and capabilityinformation of the non-AP STA that can be supported by the correspondingAP. Detailed description on the association frame format is disclosed inIEEE 802.11 8.3.3.5/8.3.3.6.

After the association procedure is carried out between the AP and thenon-AP STA, normal transmission and reception of data may be performedbetween the AP and the non-AP STA. In case the association procedurebetween the AP and the non-AP STA has failed, based on the reason whythe association procedure has failed, the association procedure may beperformed once again with the same AP, or a new association proceduremay be performed with another AP.

In case the STA is associated with the AP, the STA may be allocated withan association ID (association identifier, AID) from the AP. The AIDthat is allocated to the STA may correspond to a unique value within oneBSS, and the current AID value may correspond to any one of the valueswithin the range of 1-2007. Since 14 bits are allocated for the AID,although a maximum of 16383 bits may be used for the AID value, valueswithin the range of 2008-16383 are reserved.

FIG. 4 is a conceptual view illustrating a UL MU transmission methodaccording to an embodiment of the present invention.

In FIG. 4, a preliminary procedure for UL MU transmission of a pluralityof STAs is disclosed.

Referring to FIG. 4, when an initial access procedure (e.g., a scanningprocedure, an authentication procedure, an association procedure) of anSTA is performed, an AP and the STA may negotiate UL MU transmissioncapability. For example, the STA may transmit information regardingwhether UL MU transmission is possible to the AP through an initialaccess frame (e.g., an association request frame) used in the initialaccess procedure. The AP may transmit information regarding whether tosupport UL MU transmission through a response frame (e.g., anassociation response frame) in response to the initial access frametransmitted by the STA. If whether to support the UL MU transmission isconfirmed between the STA and the AP, the STA may be triggered by atrigger frame of the AP to transmit uplink data to the AP on the basisof the UL MU transmission.

The AP determines whether to trigger the UL MU transmission.

The AP may determine whether to trigger the UL MU transmission byconsidering a state of a BSS load, information on the number of STAssupporting UL MU transmission, or the like. For example, if the BSS loadis heavy, or if the number of STAs supporting the UL MU transmission isgreater than or equal to a threshold number, the AP may trigger the ULMU transmission.

If it is determined to trigger the UL MU transmission, the AP maycollect the buffer status information of the STA.

The AP may transmit a trigger frame 400 for random access to triggertransmission of buffer information of the STA. The trigger frame 400 forrandom access may be transmitted on a broadcast basis. The trigger frame400 for random access may also be expressed by the term “buffer statustrigger frame”.

The AP may transmit information for restricting (or determining) an STAfor transmitting buffer status information on the basis of the bufferstatus trigger frame 400 through another frame transmitted prior to thetrigger frame 400 for random access or through the trigger frame 400 forrandom access. The information for restricting (or determining) the STAfor transmitting the buffer status information on the basis of thetrigger frame 400 for random access may be expressed by the term “bufferstatus report STA determination information”. The buffer status reportSTA determination information will be described at a later time.

The STA may receive the trigger frame 400 for random access and transmita buffer status report frame 410 in response to the trigger frame 400for random access on a transmission opportunity (TXOP) acquired by thetrigger frame 400 for random access. The buffer status report frame 410may include buffer status information (or information on pending (orbuffered) uplink data) of the STA. For example, the buffer statusinformation may include a size of the pending uplink data, an accesscategory of the pending uplink data, a backoff count, or the like. Thebuffer status report frame 410 may be transmitted only by an STA whichsatisfies a condition configured based on the buffer status report STAdetermination information transmitted by the AP.

Alternatively, according to the embodiment of the present invention,when UL MU transmission is limitedly enabled in a BSS of the AP and/orwhen whether to enable it is determined, the buffer status report frame410 may be transmitted by an STA which does not satisfy the conditionconfigured based on the buffer status report STA determinationinformation. Further, when UL MU transmission is disabled in the BSS ofthe AP, the buffer status report frame 410 may not be transmitted by theSTA which satisfies the buffer status report STA determinationinformation. This will be described in detail at a later time.

Although the buffer status information of the STA may be transmittedthrough an independent frame (or stand-alone frame) such as the bufferstatus report frame 410, the STA may also transmit the buffer statusinformation in a piggyback manner. For example, the STA may transmit thebuffer status information by piggybacking on the existing data frame,control frame, or management frame transmitted on an uplink. The STA maytransmit the piggybacked uplink frame in response to the trigger frame400 for random access or transmit the buffer status information in apiggyback manner without having to use triggering based on the separatetrigger frame 400 for random access.

In this manner, the AP may acquire the buffer status information of eachof the plurality of STAs.

The AP may trigger UL MU transmission on the basis of the acquiredbuffer status information of each of the plurality of STAs. The AP maytransmit a UL MU trigger frame 420 for triggering UL MU transmission toa plurality of target STAs which will perform UL MU transmission.

The UL MU trigger frame 420 may include information on a plurality oftarget STAs which will perform UL MU transmission, information on uplinkresources for transmission of uplink data of each of the plurality oftarget STAs, information on a TXOP for transmission of uplink data ofeach of the plurality of target STAs, information on transmission timingof uplink data of each of the plurality of target STAs, or the like.

More specifically, the information on the plurality of target STAs whichwill perform UL MU transmission may include information on anassociation identifier (AID) of each of the plurality of target STAs.Further, the information on the uplink resource for transmission ofuplink data of each of the plurality of target STAs may includeinformation on a subchannel (or subband, RU) allocated to each of theplurality of target STAs and/or information on a space-time streamallocated to each of the plurality of target STAs.

Each of the plurality of target STAs may receive the UL MU trigger frame420, and may transmit uplink data through a UL MU PPDU 430 to the APthrough an uplink frame during a TXOP duration by using the allocateduplink resource.

FIG. 5 is a conceptual view illustrating a UL MU transmission procedureaccording to an embodiment of the present invention.

In FIG. 5, a method of enabling/disabling UL MU transmission of an AP isdisclosed.

Referring to FIG. 5, the AP may determine whether to enable UL MUtransmission in a BSS. Even if the UL MU transmission is supported bythe AP and an STA, the AP may determine whether to enable the UL MUtransmission in the BSS.

Information on whether to enable the UL MU transmission in the BSS maybe defined as N bits (where N is a natural number) and may betransmitted through a downlink frame (e.g., a beacon frame as a downlinkmanagement frame) or transmitted by being piggybacked on a medium accesscontrol (MAC) header of the downlink frame. The N bits indicatingwhether to enable the UL MU transmission in the BSS may be expressed bythe term “UL MU transmission enabling information (or UL MU operationmode information)”. For example, the N-bit UL MU transmission enablinginformation (or UL MU operation mode information) may indicate one ofdisabling/limited enabling/enabling of the UL MU transmission. The UL MUtransmission enabling information (or UL MU operation mode information)may be transmitted in a broadcast manner through a frame such as abeacon frame, or may be transmitted in a unicast manner through a framesuch as a piggybacked unicast downlink frame.

For example, the AP may decide a BSS load to determine whether to enablethe UL MU transmission. A BSS load state may be classified by the APinto one of a light BSS load state (or a first BSS load state), a mediumBSS load state (or a second BSS load state), and a heavy BSS load state(or a third BSS load state). The aforementioned BSS load state may bedetermined by considering the number of STAs associated with the BSS,channel utilization calculated based on a time in which a channel is ina busy state, utilization of a spatial stream (or a space-time stream),or the like.

More specifically, if the AP determines that the BSS load state is thelight BSS load state (or a first load state 510), the AP may disable ULMU transmission. The AP may transmit UL MU transmission enablinginformation indicating the disabling of the UL MU transmission. In thiscase, an STA included in a BSS may transmit uplink data to the AP on thebasis of UL SU transmission instead of the UL MU transmission.

If the AP determines that the BSS load state is the medium BSS loadstate (or a second load state 520), the AP may enable the UL MUtransmission in such a manner that the UL MU transmission is triggeredonly when a certain condition is satisfied. In other words, if the APdetermines that the BSS load state is the medium BSS load state (or thesecond load state 520), the AP may limitedly allow UL MU transmission.The AP may transmit the UL MU transmission enabling informationindicating the limited enabling of the UL MU transmission to the STAincluded in the BSS.

Only an STA which satisfies a condition configured based on bufferstatus report STA determination information may transmit buffer statusinformation to the AP, and may perform UL MU transmission. For example,if pending data is uplink data of an access category having a backoffcount greater (or smaller) than a backoff count threshold which is setbased on the buffer status report STA determination information, the STAmay transmit the buffer status information to the AP. In this case, anSTA capable of transmitting the buffer status information in the BSS maybe limited, and a target STA which will perform UL MU transmission maybe limited.

Alternatively, a target STA capable of performing UL MU transmission maybe limited by allowing the buffer status information to be transmittedonly in a piggyback manner. That is, by collecting buffer statusinformation of the STA only in a piggyback manner other than triggering(polling) based on a random access trigger frame transmitted by the AP,the STA capable of transmitting the buffer status information in the BSSmay be limited, and the target STA which will perform the UL MUtransmission may be limited.

An STA of which UL MU transmission is not triggered may perform UL SUtransmission. For example, if the buffer status information is nottransmitted and thus the UL MU transmission is not triggered by the AP,the STA may acquire a medium through channel access based on enhanceddistributed channel access (EDCA) and transmit an uplink frame to theAP.

If the AP determines that the BSS load state is the heavy BSS load state(or a third load state 530), the AP may trigger transmission of uplinkdata based on the UL MU transmission.

The AP may transmit UL MU transmission enabling information indicatingthe enabling of the UL MU transmission to the STA included in the BSS.In this case, the STA may transmit uplink data to the AP through the ULMU transmission based on the trigger of the AP.

When the STA receives the UL MU transmission enabling informationindicating the enabling of the UL MU transmission, the STA may transmitthe buffer status information to the AP without consideration of acondition configured based on the buffer status report STA determinationinformation. That is, the STA capable of transmitting the buffer statusinformation in the BSS is not limited, and the target STA which willperform the UL MU transmission is not limited.

For example, the STA may transmit the buffer status information to theAP by not considering a backoff count threshold which is set based onthe buffer status report STA determination information or by setting thebackoff count threshold to 0 or 1. The buffer status information may betransmitted by the STA by using both of a piggyback scheme and/or atriggering (or polling) scheme based on the trigger frame for randomaccess.

Even if the AP determines that the BSS load state is the heavy BSS loadstate (or a third load state 530), UL SU transmission may also beperformed. Even if the AP enables the UL transmission, the UL SUtransmission may also be performed under the following limitedcondition.

A buffer status report frame for transmission of buffer statusinformation may be triggered (or polled) by the trigger frame for randomaccess and may be transmitted by the STA on the basis of the UL SUtransmission.

Further, the STA may transmit buffer status information even if thebuffer status report frame is not triggered by the transmission of thetrigger frame for random access. A report of the buffer statusinformation is expressed by the term “unsolicited buffer status report”,and the buffer status information transmitted based on the unsolicitedbuffer status report is expressed by the term “unsolicited buffer statusinformation”

The unsolicited buffer status information may be categorized into datacorresponding to one access category (AC) among existing ACs and may betransmitted based on EDCA which uses existing AC parameters.Alternatively, the unsolicited buffer status information may betransmitted by using EDCA which uses a newly defined AC parameter bybeing defined as data corresponding to a new AC. The newly defined ACparameter may be set such that a new AC for the unsolicited bufferstatus information has a higher priority than other existing ACs (AC_BE(best effort), AC_BK (background), AC_VI (video), AC_VO (voice)).

The unsolicited buffer status information may be transmitted when a setspecific timer expires The set specific timer is a timer for setting atime duration for transmission of a triggered buffer status reportframe, and may be expressed by the term “max polling timer” or “maxtrigger timer”. That is, the unsolicited buffer status report may beperformed on a time resource except for a time resource for transmissionof a buffer status report frame transmitted on a trigger basis.

The UL MU transmission method for a case where the AP determines thatthe BSS load state is the heavy BSS load state (or the third load state)is additionally disclosed in FIG. 7.

FIG. 6 is a conceptual view illustrating buffer status report STAdetermination information according to an embodiment of the presentinvention.

In FIG. 6, threshold information transmitted through buffer statusreport STA determination information 600 is disclosed. Whether toperform transmission of the buffer status report frame of the STA (orreport of the buffer status information) may be determined based on thebuffer status report STA determination information 600 transmitted by anAP. The buffer status report STA determination information 600 may betransmitted by the AP through a downlink frame (e.g., a beacon frame, anassociation response frame, a trigger frame for random access, etc.).

Referring to FIG. 6, the buffer status report STA determinationinformation 600 may include at least one of back-off count information610, AC information 620, and queue size information 630.

The backoff count information 610 may include information on a thresholdof a backoff count for transmission of the buffer status report frame.The STA may have pending uplink data corresponding to each of aplurality of access categories, and a separate backoff countcorresponding to each of the plurality of access categories may be setfor transmission of the pending uplink data.

If a back-off count for transmission of buffered data corresponding to aspecific access category is greater than the threshold of the back-offcount set by the back-off count information 610, the STA may transmitbuffer status information related to the specific access category to theAP through the buffer status report frame. If the backoff count fortransmission of the buffered data corresponding to the specific accesscategory is less than or equal to the threshold of the backoff count setby the backoff count information 610, the STA may not transmit thebuffer status information related to the specific access category to theAP through the buffer status report frame.

According to another embodiment of the present invention, on thecontrary, if the backoff count for transmission of the buffered datacorresponding to the specific access category is less than or equal tothe threshold of the backoff count set by the backoff count information610, the STA may transmit the buffer status information related to thespecific access category to the AP through the buffer status reportframe. If the back-off count for transmission of the buffered datacorresponding to the specific access category is greater than thethreshold of the back-off count set by the back-off count information610, the STA may not transmit the buffer status information related tothe specific access category to the AP through the buffer status reportframe.

The AC information 620 may include information on the access category tobe reported by the STA. The AC information 620 may be used to requestonly buffer status information related to the specific access categoryamong a plurality of access categories. For example, the AC information620 may include four bits, and each of the four bits may indicate anaccess category for requesting the buffer status information to the STA.Among the four bits, a bit 1 may enable a report of buffer statusinformation corresponding to AC_BE (best effort), a bit 2 may enable areport of buffer status information corresponding to AC_BK (background),a bit 3 may enable a report of buffer status information correspondingto AC_VI (video), and a bit 4 may enable a report of buffer statusinformation corresponding to AC_VO (voice). The STA may transmit onlythe buffer status information related to the enabled access categorybased on AC information to the AP through the buffer status reportframe.

The queue size information 630 may include information on a thresholdsize of uplink data for transmission of the buffer status information.The STA may determine whether there is uplink data having a queue size(or data size) greater than a threshold size set by the queue sizeinformation 630 among pending uplink data corresponding to each of theplurality of access categories of the STA. The STA may transmit only thebuffer status information related to the access category correspondingto the uplink data having the queue size greater than the threshold sizeto the AP through the buffer status report frame.

According to another embodiment of the present invention, on thecontrary, the STA may determine whether there is data having a queuesize smaller than or equal to the threshold size set by the queue sizeinformation 630 among the buffered data corresponding to each of theplurality of access categories of the STA. The STA may transmit only thebuffer status information related to the access category correspondingto data having the queue size smaller than or equal to the thresholdsize to the AP through the buffer status report frame.

In FIG. 4, it is assumed that the buffer status information istransmitted through the buffer status report frame when the thresholdinformation transmitted through the buffer status report STAdetermination information 600 is satisfied. However, if the thresholdinformation transmitted through the buffer status report STAdetermination information 600 is satisfied, the buffer statusinformation may be transmitted through an uplink frame in which thebuffer status information is piggybacked.

FIG. 7 is a conceptual view illustrating a method of triggering a bufferstatus report frame of an STA according to an embodiment of the presentinvention.

In FIG. 7, a UL MU transmission method is disclosed when an APdetermines that a BSS load state is a heavy BSS load state (or a thirdload state).

Referring to FIG. 7, the AP may assign a TXOP for transmission of abuffer status report frame to a plurality of unspecific STAs other thana specific STA. The unspecific STA may be expressed by the term“anonymous STA”.

The TXOP may be expressed by the term “random access TXOP” 720 or“broadcast TXOP” or “buffer status report TXOP”. The AP may allocate arandom access TXOP 700 for transmission of a buffer status report frame740 of at least one unspecific STA. At least one unspecific STA maytransmit the buffer status report frame 740 through the allocated randomaccess TXOP 700. The TXOP may be a time resource and/or frequencyresource allocated for transmission of data. The random access TXOP 700may indicate a frequency resource/time resource for transmission of thebuffer status report frame 740.

The AP may transmit information for allocation of the random access TXOP720. The information for allocation of the random access TXOP 720 mayinclude at least one of a start offset of the TXOP and information onthe frequency resource/time resource to which the TXOP is allocated.Information for allocation of the random access TXOP 720 may betransmitted through a downlink management/control frame (e.g., a beaconframe, a broadcast clear to send (CTS) frame, a trigger frame, a newcontrol frame, etc.) to be broadcast.

The unspecific STA may be allocated the random access TXOP 720 on thebasis of information for allocation of the random access TXOP 720 andmay transmit the buffer status report frame 740 through random accesschannel access.

That is, the AP may transmit a trigger frame for random access andreceive at least one buffer status report frame from at least one STA onthe random access TXOP 720 which is configured based on the triggerframe for random access. The random access TXOP 720 is allocated toattempt channel access for transmission of a buffer status report frameof each of the plurality of unspecific STAs, and each of the at leastone STA may be an STA which has succeeded in the channel access amongthe plurality of unspecific STAs. More specifically, by consideringinformation on the number of frequency resource units allocated for therandom access TXOP 720, at least one STA may be determined based on abackoff procedure in the allocated frequency resource unit.

According to the embodiment of the present invention, the unspecific STAmay transmit the buffer status report frame 740 during the random accessTXOP 720 if the following condition is satisfied.

The unspecific STA may transmit the buffer status report frame 740 onthe random access TXOP 720 if a time set by a specific timer such as amaximum poll timer expires.

Alternatively, a parameter (e.g., a contention window value forselecting a backoff count, etc.) used in the backoff procedure fortransmission of the buffer status report frame through random access maybe defined as a separate parameter unlike in a backoff parameter used inthe existing data transmission. In this case, if the backoff countrelated to transmission of the buffer status report frame 740 becomeszero, the unspecific STA may transmit the buffer status report frame740.

Alternatively, if the backoff count related to transmission of thebuffered data is less than the threshold of the backoff count, theunspecific STA may transmit the buffer status report frame 740. On thecontrary, if the backoff count related to transmission of the buffereddata is greater than or equal to the threshold of the backoff count, theunspecific STA may transmit the buffer status report frame 740. Thethreshold of the backoff count may be set by backoff count informationincluded in the buffer status report STA determination information.

FIG. 8 is a conceptual view illustrating a method of triggering a bufferstatus report frame of an STA according to an embodiment of the presentinvention.

In FIG. 8, a channel access method of the STA on a random access TXOP850 is disclosed. If a plurality of STAs perform channel access throughrandom access on the random access TXOP 850 for transmission of a bufferstatus report frame, a collision may occur between buffer status reportframes transmitted by the plurality of STAs.

Referring to FIG. 8, a plurality of frequency resource units(subchannels, subbands, or RUs) may be allocated as the random accessTXOP 850 after a trigger frame 800 for random access is transmitted.Hereinafter, it is assumed that each of an RU1, an RU2, an RU3, and anRU4 is allocated as a transmission resource for transmission of a bufferstatus report frame. The trigger frame 800 for random access maytransmit information regarding the allocated time resource/frequencyresources as described above.

The plurality of STAs which have received the trigger frame 800 forrandom access may determine whether the buffer status report frame istransmitted through each of the plurality of frequency resource units.For example, each of the plurality of STAs may determine whether totransmit the buffer status report frame on the random access TXOP 850through a backoff procedure. More specifically, the backoff proceduremay be performed in a frequency resource unit. If the number offrequency resource units is 4 as in the above assumption, an STA whichhas selected 0 to 3 as the backoff count for the buffer status reportmay transmit the buffer status report frame on the random access TXOP850 configured based on the trigger frame 800 for random access. The STAwhich has selected 0 to 3 as the backoff count may transmit the bufferstatus report frame on the random access TXOP 850 on the basis of UL MUtransmission.

On the contrary, an STA which has selected a value greater than or equalto 4 as the remaining backoff counts cannot transmit the buffer statusreport frame on the random access TXOP 850.

Each of four STAs (STA1, STA2, STA3, and STA4) which have selected 0 to3 as the back-off counts may randomly determine an RU which willtransmit the buffer status report frame. Each of the four STAs (STA1,STA2, STA3, and STA4) may transmit the buffer status report frame on therandomly determined RU. Ideally, the STA1 may transmit a buffer statusreport frame1 810 through the RU1, the STA2 may transmit a buffer statusreport frame2 820 through the RU2, the STA3 may transmit a buffer statusreport frame3 830 through the RU3, and the STA 4 may transmit a bufferstatus report frame4 840 through the RU 4.

According to another embodiment of the present invention, the STA mayrandomly perform channel access to transmit the buffer status reportframe on the random access TXOP 850 without the backoff procedure.

Although the procedure of setting the broadcast TXOP based on thetrigger frame for random access is described in FIG. 7 and FIG. 8, theaforementioned UL MU transmission procedure may also be performed foranother trigger frame for triggering another frame other than thetrigger frame for random access.

FIG. 9 is a conceptual view illustrating a DL MU PPDU format accordingto an embodiment of the present invention.

In FIG. 9, a DL MU PPDU format transmitted based on OFDMA by an AP isdisclosed according to the embodiment of the present invention. The DLMU PPDU format may be used to deliver a trigger frame for random access,a UL MU trigger frame, or the like.

Referring to an upper portion of FIG. 9, a PHY header of a DL MU PPDUmay include a legacy-short training field (L-STF), a legacy-longtraining field (L-LTF), a legacy-signal (L-SIG), a highefficiency-signal A (HE-SIG A), a high efficiency-signal-B (HE-SIG B), ahigh efficiency-short training field (HE-STF), a high efficiency-longtraining field (HE-LTF), and a data field (or a MAC payload). The PHYheader may be divided into a legacy part before the L-SIG and a highefficiency (HE) part after the L-SIG.

An L-STF 900 may include a short training orthogonal frequency divisionmultiplexing (OFDM) symbol. The L-STF 900 may be used for framedetection, automatic gain control (AGC), diversity detection, and coarsefrequency/time synchronization.

An L-LTF 910 may include a long training OFDM symbol. The L-LTE 910 maybe used for fine frequency/time synchronization and channel prediction.

An L-SIG 920 may be used to transmit control information. The L-SIG 920may include information for a data rate and a data length.

An HE-SIG A 930 may include information for indicating an STA forreceiving a DL MU PPDU. For example, the HE-SIG A 930 may include anidentifier of a specific STA (or AP) for receiving the PPDU andinformation for indicating a group of the specific STA. Further, if theDL MU PPDU is transmitted based on orthogonal frequency divisionmultiple access (OFDMA) or multiple input multiple output (MIMO), theHE-SIG A 930 may also include resource allocation information forreceiving the DL MU PPDU of the STA.

Further, the HE-SIG A 930 may include color bits information for BSSidentification, bandwidth information, a tail bit, a CRC bit, modulationand coding scheme (MCS) information for an HE-SIG B 940, symbol countinformation for the HE-SIG B 940, and cyclic prefix (CP) (or guardinterval (GI)) length information.

The HE-SIG B 940 may include a length of physical layer service dataunit (PSDU) for each STA, information regarding modulation and codingscheme (MCS), a tail bit, or the like. Further, the HE-SIG B 940 mayinclude information for the STA for receiving the PPDU and OFDMA-basedresource allocation information (or MU-MIMO information). If theOFDMA-based resource allocation (or MU-MIMO related information) isincluded in the HE-SIG B 940, resource allocation information may not beincluded in the HE-SIG A 930.

A field prior to the HE-SIG B 940 on the DL MU PPDU may be transmittedin a duplicated form in each of different transmission resources. Incase of the HE-SIG B 940, the HE-SIG B 940 transmitted in somesubchannels (e.g., subchannel 1, subchannel 2) may be an independentfield containing individual information, and the HE-SIG B 940transmitted in the remaining subchannels (e.g., subchannel 3, subchannel4) may have a format in which the HE-SIG B 940 transmitted in othersubchannels (e.g., subchannel 1, subchannel 2)) is duplicated.Alternatively, the HE-SIG B 940 may be transmitted on all transmissionresources in an encoded form. A field next to the HE-SIG B 940 mayinclude individual information for each of the plurality of STAs forreceiving the PPDU.

An HE-STF 950 may be used to improve automatic gain control estimationin an MIMO environment or an OFDMA environment.

More specifically, an STA1 may receive an HE-STF1 transmitted through asubband1 from the AP, and may decode a data field1 by performingsynchronization, channel tracking/prediction, and AGC. Similarly, anSTA2 may receive an HE-STF2 transmitted through a subband2 from the AP,and may decode a data field2 by performing synchronization, channeltracking/prediction, and AGC. An STA3 may receive an HE-STF3 transmittedthrough a subband3 from the AP, and may decode a data field3 byperforming synchronization, channel tracking/prediction, and AGC. AnSTA4 may receive an HE-STF4 transmitted through a subband4 from the AP,and may decode a data field4 by performing synchronization, channeltracking/prediction, and AGC.

An HE-LTF 960 may be used to estimate a channel in the MIMO environmentor the OFDMA environment.

A size of IFFT applied to the HE-STF 950 and a field next to the HE-STF950 may be different from a size of IFFT applied to a field prior to theHE-STF 950. For example, the size of IFFT applied to the HE-STF 950 andthe field next to the HE-STF 950 may be four times greater than the sizeof IFFT applied to the field prior to the HE-STF 950. The STA mayreceive the HE-SIG A 930, and may be instructed to receive a downlinkPPDU on the basis of the HE-SIG A 930. In this case, the STA may performdecoding on the HE-STF 950 and the field next to the HE-STF 950 on thebasis of a changed FFT size. On the contrary, if the STA is notinstructed to receive the downlink PPDU on the basis of the HE-SIG A930, the STA may stop decoding and may configure a network allocationvector (NAV). A cyclic prefix (CP) of the HE-STF 950 may have a sizegreater than a CP of another field, and for this CP duration, the STAmay perform decoding on the downlink PPDU by changing the FFT size.

FIG. 10 is a conceptual view illustrating transmission of a UL MU PPDUaccording to an embodiment of the present invention.

Referring to FIG. 10, a plurality of STAs may transmit the UL MU PPDU onthe basis of UL MU OFDMA to an AP. In FIG. 10, only a UL MU PPDU1transmitted by an STA1 is disclosed for example. The UL MU PPDU may beused to deliver a buffer status report frame and a UL MU data frame.

An L-STF 1000, an L-LTF 1010, an L-SIG 1020, an HE-SIG A 1030, and anHE-SIG B 1040 may perform the function disclosed in FIG. 9. Informationincluded in a signal field (the L-SIG 1020, the HE-SIG A 1030, and theHE-SIG B 1040) may be generated based on information included in asignal field of a received DL MU PPDU.

The STA1 may perform uplink transmission through a full bandwidth untilthe HE-SIG B 1040, and may perform uplink transmission through anallocated bandwidth starting from an HE-STF 1050. As described above inFIG. 8, the STA1 may select a frequency resource (e.g., subband 1) forrandomly transmitting a buffer status report frame, and may transmit thebuffer status report frame on the selected frequency resource. Thebuffer status report frame may be included in the data field 1.

It may be assumed that a subband 1 to a subband 4 are used as a randomaccess TXOP and the STA1 to the STA4 are STAs capable of transmittingthe buffer status report frame on the random access TXOP. In an idealcase, the STA2 may transmit a data field2 containing an HE-STF2 and anSTA2's buffer status report frame through a subband2 allocated by theAP, the STA3 may transmit a data field3 containing an HE-STF3 and anSTA3's buffer status report frame through a subband3 allocated by theAP, and the STA4 may transmit a data field4 containing an HE-STF4 and anSTA4's buffer status report frame4 through a subband4 allocated by theAP.

FIG. 11 is a block diagram illustrating a wireless device according toan embodiment of the present invention.

Referring to FIG. 11, an AP 1100 includes a processor 1110, a memory1120, and a radio frequency (RF) unit 1130.

The RF unit 1130 may be coupled to the processor 1110 totransmit/receive a radio signal.

The processor 1110 may implement the functions, procedures, and/ormethods proposed in the present invention. For example, the processor1110 may be configured to perform an operation of the AP according tothe aforementioned embodiment of the present invention. The processormay perform the operation of the AP disclosed in the embodiment of FIG.1 to FIG. 10.

For example, the processor 1110 may be configured to transmit a triggerframe for random access, and receive at least one buffer status reportframe from at least one station (STA) on a random access transmissionopportunity (TXOP) configured based on the trigger frame for randomaccess. The random access TXOP may be allocated to attempt channelaccess for transmission of the buffer status report frame of each of aplurality of unspecific STAs, and each of the at least one STA may be anSTA which has succeeded in the channel access among the plurality ofunspecific STAs. The at least one STA may be determined based on abackoff procedure in an allocated frequency resource unit by consideringinformation regarding the number of frequency resource units allocatedfor the random access TXOP.

An STA 1150 includes a processor 1160, a memory 1170, and a radiofrequency (RF) unit 1180.

The RF unit 1180 may be coupled to the processor 1160 totransmit/receive a radio signal.

The processor 1160 may implement the functions, procedures, and/ormethods proposed in the present invention. For example, the processor1160 may be configured to perform an operation of the STA according tothe aforementioned embodiment of the present invention. The processor1160 may perform the operation of the STA disclosed in the embodiment ofFIG. 1 to FIG. 10.

For example, the processor 1160 may receive a trigger frame for randomaccess, and may transmit at least one buffer status report frame on arandom access transmission opportunity (TXOP) configured based on thetrigger frame for random access.

The processors 1110 and 1160 may include application-specific integratedcircuits (ASICs), other chipsets, logical circuits, data processingdevices, and/or converters for mutually converting a baseband signal anda radio signal. The memories 1120 and 1170 may include a read-onlymemory (ROM), a random access memory (RAM), a flash memory, a memorycard, a storage medium and/or other storage devices. The RF units 1130and 1180 may include at least one antenna to transmit and/or receive theradio signal.

When the above-described embodiment is implemented in software, theabove-described scheme may be implemented using a module (process orfunction) which performs the above function. The module may be stored inthe memories 1120 and 1170 may be and executed by the processors 1110and 1160. The memories 1120 and 1170 may be disposed to the processors1110 and 1160 internally or externally and connected to the processors1110 and 1160 using a variety of well-known means.

What is claimed is:
 1. A method of transmitting a buffer status report(BSR) in a wireless local area network (WLAN), the method performed by astation (STA) and comprising: receiving, from an access point (AP),first control information related to the BSR; receiving, from the AP, atrigger frame which triggers uplink multiple user (UL-MU) transmission,wherein the trigger frame includes second control information related tofrequency resources used for a plurality of stations (STAs); and afterreceiving the trigger frame, determining whether to transmit the BSR ofthe STA to the AP on at least one of the frequency resources based onthe first control information and a back-off count of the STA, whereinthe BSR of the STA is determined to be transmitted when the back-offcount of the STA becomes zero.
 2. The method of claim 1, wherein theback-off count decrements to zero through a back-off procedure performedby the STA.
 3. The method of claim 1, wherein the frequency resourcesare non-dedicated frequency resources, which are not exclusivelyallocated by the AP to a specific STA, and the at least one of thefrequency resources is randomly selected by the STA when the back-offcount of the STA becomes zero.
 4. A station (STA) in a wireless localarea network (WLAN), comprising: a memory storing a signal received froman access point (AP); and a processor operatively coupled to the memory,wherein the processor is configured to: receive, from the AP, firstcontrol information related to a buffer status report (BSR); receive,from the AP, a trigger frame which triggers uplink multiple user (UL-MU)transmission, wherein the trigger frame includes information related tofrequency resources used for a plurality of stations (STAs); and afterreceiving the trigger frame, determine whether to transmit the BSR ofthe STA to the AP on at least one of the frequency resources based onthe first control information and a back-off count of the STA, whereinthe BSR of the STA is determined to be transmitted when the back-offcount of the STA becomes zero.
 5. The STA of claim 4, wherein theback-off count decrements to zero through a back-off procedure performedby the STA.
 6. The STA of claim 4, wherein the frequency resources arenon-dedicated frequency resources, which are not exclusively allocatedby the AP to a specific STA, and the at least one of the frequencyresources is randomly selected by the STA when the back-off count of theSTA becomes zero.